<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <script>

    //  const docW = window.innerWidth;
    //  console.log('docW', docW)
    //  const font = docW / 750 * 100; // .5  <12px
    //  document.querySelector('html').style.fontSize = font + 'px';
    // // 375 1rem = 50

    // 750 1rem = 100px
  </script>
  <style>
    *{
      margin:0;
      padding:0;
    }
    html {
      font-size: calc(100 / 750 * 100vw);
    }
    /* ui 750px  */
    .box {
      width: 7.5rem;
      height: 5rem;
      background: lightblue;
    }
  </style>
</head>
<body>

  <div class="box"></div>

</body>
</html>
